In recent years, an image projector using a metal halide lamp, halogen lamp, Xenon arc lamp, high-pressure mercury discharge lamp, or the like (referred to as “a lamp or the like,” hereinafter,) as a light source have become common. For the image projector of this type, output light from the lamp or the like being used as a light source is separated into red light (long-wavelength light), green light (intermediate-wavelength light), and blue light (short-wavelength light) by means of a wavelength selective mirror. Each separated light of such colors is modulated separately by a liquid crystal panel. Then, the separated and modulated light is multiplexed by a dichroic prism and projected onto a screen by a projector lens. This emerges a color image on the screen.
It is unfortunate, however, that the above said lamp or the like has relatively short lifetime. Hence, when the above said lamp or the like is used as a light source, it is a hassle to maintain the light source. In addition, three primary colors are created by separating white light from above said lamp or the like by means of above mentioned method. So it complicates an optical system of the projector. Owing to the optical property of the wave selective mirror, the light separated by the wave selective mirror exhibits relatively broad spectrum width. Accordingly, the color reproduction region of the projector is confined within a restricted narrow area and, hence, it is difficult to deliver a vivid pure color. Moreover, the image projector of this type has an issue of low light use efficiency.
In order to overcome such issues from which the image projector using the above lamp or the like suffers, many trials in which a laser light source has been used as a light source have been gone through in recent years. A laser light source operates longer than conventional white light sources and have high energy efficiency, and, in addition, is advantageous in terms of light use efficiency due to its excellent directivity of laser light. Furthermore, good monochromaticity of laser light may make a color reproduction region of the image projector wider than that of the above said type of image projector.
When the laser light source is used in the image projector as described above, however, there is a problem that inherent coherency (nature of being prone to interfere) of laser light causes speckle noise to be found in an image and, as a consequence, it deteriorates the picture quality of the projected image. The term speckle noise here includes a phenomenon wherein granulated intensity distribution of light is found on a viewing plane. The light is a bundle of coherent light from the laser light source and is scattered at several locations on an object surface. On the viewing plane, a wavefront of the coherent light having scattered at a certain location of the object surface and another wavefront of the coherent light having scattered at another location nearby the above said certain location interfere with each other. As a consequence, the granulated intensity distribution is found on the viewing plane. For practical use of the image projector that uses a laser as a light source, there still remains the serious problem of reducing the generation of the speckle noise.
So as to suppress the speckle noise, the exposure illumination apparatus of JP07-297111-A (Patent Document 1) that uses laser light includes a diffuser plate that can rotate in its optical system such that the diffuser plate may turn coherent light into incoherent light, for example.
Moreover, the projection display apparatus of JP06-208089 (Patent Document 2) that uses laser light includes a movable diffuser plate (that can rotate and/or vibrate etc.) in its optical system such that the diffuser plate may turn coherent light into incoherent light.
[Patent Document 1] JP 07-297111-A
[Patent Document 2] JP 06-208089-A